The founding of the hill fort is related to the mining activity in the area.

San Isidro Hill Fort is located on the ridge of peaks marking the boundary between the boroughs of Pesoz and San Martín de Oscos, at an elevation of 600 metres. This is not its only unique feature, as it presents a kind of fortification that is exceptional in the region: standing stones, also known as "Friesian horses".

The ruins are spread over an area of just under a hectare, much of which is occupied by the fortifications. As they approach the hilltop, visitors can observe the succession of ditches surrounding the entrance to the site. On intermediate parapets can be found the "standing stones", successive lines of vertical slabs of slate, very effective in hindering the passage of the enemy. Behind these rise a 3-m-wide monumental wall, whose layout allows a full tour of the site. The original depth of the ditches and the base of the parapets can be appreciated in the excavated areas, as well as the walkway around the inside of the wall and one or another of the buildings discovered inside the enclosure.

The founding of the hill fort seems to be related to the establishment of military units and control of the gold mines that Rome mined during the 1st and 2nd centuries AD.